The proposed study is a multi-institutional, 5-year, longitudinal study of bone disease in children with MRS I, II, and VI. The overall goal is to characterize bone disease in MRS I, II, and VI by measurements of bone architecture, density, strength, metabolism, and mobility. This will be done using the techniques of dual energy x-ray absorptiometry (DXA), peripheral quantitative computer tomography (pQCT), and serum markers of bone metabolism. The specific aims are: 1) To characterize the bone health and bone architecture, density, strength, and mobility, and to document the natural progression of bone disease using DXA, peripheral quantitative computer tomography (pQCT), biomarkers of bone turnover, and tests of mobility. 2) In a subset of the population (height < -2 SDS) we aim to assess the efficacy of human growth hormone (hGH) on increasing growth velocity, and the impact on bone health and architecture, mobility, and neurocognitive outcomes during 2 study years. 3) To standardize measurements of DXA, pQCT, and mobility between the University of Minnesota and the University of Utah such that these measures could then be used in other network, multi-center therapeutic trials.